This invention relates to novel silver compounds that can be used as a source of reducible silver ions in thermally developable imaging materials. The invention also includes imaging compositions and methods of making the silver compounds. In particular, the invention relates to thermographic and photothermographic materials containing these silver compounds.
Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225xc2x0 C. resulting in the formation of an image.
Silver-containing photothermographic imaging materials are photosensitive materials that are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as xe2x80x9cdry silverxe2x80x9d materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms (Ag0)n, also known as silver specks, clusters, nuclei or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [D. H. Klosterboer, Imaging Processes and Materials, (Neblette""s Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin-situ,xe2x80x9d for example by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in-situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Yu. E. Usanov et al., J. Imag. Sci. Tech. 1996, 40, 104]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Yu. E. Usanov et al., International Conference on Imaging Science, Sep. 7-11, 1998, pp.67-70).
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex-situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to and be present during the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acids.xe2x80x9d Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles, and silver acetylides may also be used. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloper,xe2x80x9d may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called xe2x80x9cinstant photography,xe2x80x9d the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
While a number of useful thermographic and photothermographic materials are available in the market and described in the art for medical and graphic arts use, there is a continuing need for improving the reactivity of the imaging composition in such materials to provide reducible silver ions. In particular, there is a need for imaging materials that utilize non-photosensitive silver compounds that can be imaged and/or developed at lower temperatures while providing high Dmax, good image tone, quality, and stability.
The present invention provides a core-shell silver compound comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises a organic silver coordinating ligand.
This invention also provides a composition comprising:
a) a core-shell silver compound comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand, and
b) a non-photosensitive non-core-shell silver salt.
In another embodiment, this invention provides a composition comprising:
a) a first core-shell silver compound comprising a first primary core comprising one or more photosensitive silver halides, and a first shell at least partially covering the first primary core, wherein the first shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand, and
b) a second core-shell silver compound comprising a second primary core comprising one or more photosensitive silver halides, and a second shell at least partially covering the second primary core, wherein the second shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand,
the first and second core-shell silver compounds differing in composition in either their primary cores and/or shells.
In one embodiment, the composition further comprises a binder. In another embodiment, the composition comprises a reducing agent composition for reducible silver ions. In yet another embodiment, the composition further comprises a photocatalyst. A preferred photocatalyst is a photosensitive silver halide.
Further, a thermally developable emulsion comprises:
a) a source of non-photosensitive silver ions comprising a core-shell silver compound comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand,
b) a reducing composition for the non-photosensitive silver ions, and
c) a binder.
In one embodiment, the thermally developable emulsion further comprises a photocatalyst. A preferred photocatalyst is a photosensitive silver halide.
In addition, a thermally developable imaging material comprises a support having thereon one or more imaging layers comprising:
a) a source of non-photosensitive silver ions comprising a core-shell silver compound comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand,
b) a reducing composition for the non-photosensitive silver ions, and
c) a binder.
In a preferred embodiment, this invention provides a photothermographic material comprising a support having thereon one or more layers comprising:
a) a source of non-photosensitive silver ions comprising a core-shell silver compound comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand,
b) a reducing composition for the non-photosensitive silver ions,
c) a binder, and
d) a photocatalyst.
This invention also provides a method of making the core-shell silver compounds described above, the method comprising mixing a core-shell photosensitive silver halide with one or more ammonium or alkali metal salts of organic silver coordinating ligands for sufficient time to form the core-shell silver compound. comprising a primary core comprising one or more photosensitive silver halides, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts comprised of one or more organic silver coordinating ligands.
The present invention also provides a core-shell silver compound comprising a primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand.
This invention also provides a composition comprising:
a) a core-shell silver compound comprising a primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand, and
b) a non-photosensitive non-core-shell silver salt.
In another embodiment, this invention provides a composition comprising:
a) a first core-shell silver compound comprising a first primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a first shell at least partially covering the first primary core, wherein the first shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand, and
b) a second core-shell silver compound comprising a second primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a second shell at least partially covering the second primary core, wherein the second shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand,
the first and second core-shell silver compounds differing in composition in either their primary cores and/or shells.
In one embodiment, the composition further comprises a photocatalyst. A preferred photocatalyst is a photosensitive silver halide.
Further, this invention also provides a thermally developable emulsion comprising:
a) a source of non-photosensitive silver ions comprising a core-shell silver compound comprising a primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which non-photosensitive silver salts comprises an organic silver coordinating ligand,
b) a reducing composition for the non-photosensitive silver ions, and
c) a binder.
In addition, a thermally developable imaging material comprises a support having thereon one or more imaging layers comprising:
a) a source of non-photosensitive silver ions comprising a core-shell silver compound comprising a primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which non-photosensitive silver salts comprises an organic silver coordinating ligand,
b) a reducing composition for the non-photosensitive silver ions, and
c) a binder.
This invention also provides a method of making core-shell silver compounds, the method comprising:
mixing a core-shell non-photosensitive metal salt comprising a primary core comprising one or more non-photosensitive inorganic metal salts or non-silver-containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts,
with one or more ammonium or alkali metal salts of organic silver coordinating ligands for sufficient time to form a core-shell silver compound comprising a primary core comprising one or more non-photosensitive metal salts, and a shell at least partially covering said primary core, which shell comprises one or more non-photosensitive silver salts comprised of said one or more organic silver coordinating ligands.
Also provided by this invention is a surfactant-free composition comprising a non-photosensitive organic silver salt comprising an organic coordinating ligand, the organic silver salt having an average particle size of less than or equal to 0.5 xcexcm.
A thermally developable composition comprises:
a) the surfactant-free composition noted above containing a non-photosensitive organic silver salt having an average particle size of less than or equal to 0.5 xcexcm, and
b) a reducing agent for the non-photosensitive silver salt.
In addition, a thermally developable imaging material comprises a support having thereon one or more imaging layers comprising:
a) the surfactant-free composition noted above containing a non-photosensitive organic silver salt having an average particle size of less than or equal to 0.5 xcexcm,
b) a reducing composition for the non-photosensitive silver ions, and
c) a binder.
Further, a photothermographic material comprises a support having thereon one or more layers comprising:
a) the surfactant-free composition noted above containing a non-photosensitive organic silver salt having an average particle size of less than or equal to 0.5 xcexcm,
b) a reducing composition for the non-photosensitive silver ions,
c) a binder, and
d) a photocatalyst.
This invention also provides a method of making the non-photosensitive organic silver salts described above, the method comprising mixing a non-photosensitive silver halide with one or more ammonium or alkali metal salts of an organic silver coordinating ligand for a sufficient time to form the organic silver salt. This method can be used to prepare the non-photosensitive organic silver salts having an average particle size of less than or equal to 0.5 xcexcm, described above.
The present invention further provides organic silver compounds comprising one or more non-photosensitive silver salts, each of which silver salts comprises an organic silver coordinating ligand, the organic silver compound formed by reaction of a silver halide with one or more ammonium or alkali metal salts of an organic silver coordinating ligand for a sufficient time to form the organic silver compound.
In another embodiment, the present invention provides a method comprising imagewise exposing the thermally developable material of this invention to thermal energy to form a visible image.
In another embodiment, the present invention provides a method comprising:
A) imagewise exposing the photothermographic material of this invention to electromagnetic radiation to which the photocatalyst (for example, a photosensitive silver halide) of the material is sensitive, to form a latent image, and
B) simultaneously or sequentially, heating the exposed material to develop the latent image into a visible image.
Thermographic and photothermographic materials incorporating both the novel core-shell silver compounds and the novel non-core-shell compounds of this invention as the non-photosensitive source of reducible silver ions can provide images with desired image stability, Dmax, and tone. They can be imaged and/or developed at lower temperatures.
The novel core-shell silver compounds of this invention are prepared using a novel and simple method whereby core-shell photosensitive silver halide grains are mixed with a salt comprising an organic silver coordinating ligand (such as a carboxylate or a benzotriazolate). The organic silver coordinating ligand reacts with the silver in the xe2x80x9cshellxe2x80x9d portion of the silver halide grains to provide a xe2x80x9cshellxe2x80x9d of non-photosensitive silver salt around the unreacted core of silver halide. The novel core-shell silver compounds so formed have different reactivity and crystal morphology from core-shell silver compounds prepared by previously used methods.
Similarly, the novel non-core-shell silver compounds of this invention are also prepared using a novel and simple method whereby non-photosensitive silver halide grains are mixed with a salt comprising an organic silver coordinating ligand (such as a carboxylate or a triazolate). The organic silver coordinating ligand replaces the halide in the silver halide grains to provide a non-photosensitive silver salt. The novel non-core-shell silver compounds so formed have different reactivity and crystal morphology from core-shell silver compounds prepared by previously used methods.
Additionally, the novel core-shell silver compounds of this invention are prepared using a novel and simple method whereby non-photosensitive metal salt grains are mixed with a salt comprising an organic silver coordinating ligand (such as a carboxylate or a triazolate). The organic silver coordinating ligand replaces the anion of the metal salt to provide a non-photosensitive silver salt. The novel -core-shell silver compounds so formed have different reactivity and crystal morphology from core-shell silver compounds prepared by previously used methods.
The invention provides a means for providing predetermined organic silver salts with varying reactivity and unique imaging properties, particularly at the core-shell interface. Thus, thermally imageable materials can be prepared having specific predetermined properties.